1. Field of Invention
This invention relates to a multichannel all-optical ring network, more particularly to a bidirectional multichannel optical ring network using WDM techniques.
2. Description of the Related Art
In present communication systems, all-optical networks receive a great deal of attention. The transmission capacity of future communication networks is anticipated to be dependent on which types of communications will be serviced. Many services which have large information data bases such as, three dimensional pictures and remote medical services are gradually becoming popular, and so a tremendous quantity of information is required to be communicated. In order to maximize the data transfer rate and through put of the network, various multiplexing techniques such as wavelength division multiplexing (WDM), optical frequency division multiplexing (OFDM), and optical time division multiplexing (OTDM) have been proposed. Optical switching and repeating devices have been developed for the optical processing in order to overcome electronic speed bottlenecks in the network.
In a similar effort, Rance M. Fortenberry et al, proposed an optically transparent node for use within a photonic LAN (Local Area Network) or MAN (Metropolitan Area Network), which has several advantages over an equivalent electronic network, including flexibility of data format and the ability to avoid electronic speed bottlenecks in their paper entitled "Optically transparent node for a multiple-bit-rate photonic Packet-switched local area network", OFC/IOOC '93 Technical Digest (1993), pp 21-23.
Since the introduction of optical devices and technology using a wide bandwidth of a single mode fiber for telecommunication applications, the study of the architecture and the protocol of all-optical networks have been a major concern. Among these, ring topology optical networks are of great interest.
Generally, an optical ring network is widely used in B-ISDN, local area networks (LAN's) and customer premise networks (CPN's). Such an network is known from the articles by N. Shimosaka et al. entitled "Wavelength-addressed Optical Network using an ATM cell-based access scheme", OFC/IOOC '93 Technical Digest (1993), pp 49-50; and by J. D. Shin et al. entitled "Photonic packet-switching device for WDM-based optical ring networks", SPIE, vol. 1974, pp 221-229.
In the J. D. Shin et al, the optical ring network consists of a number of nodes linked by optical ring cables. A source node transmits a cell or a packet to a destination node through the ring cables, the cell including an address field and an information content of a cell, i.e. a payload in turn, and the address field indicating the address of the destination node. When the cell is received by a node, if the received address field, i.e. a destination address doesn't match the node's own address, the node is regarded as an intermediate node between the source node and destination node, so that the node transmits the received cell to the following node through the ring cables. Otherwise, if the destination address matches the node's own address, the node is regarded as a destination node and stores the received cell in a receive buffer.
FIG. 1 shows a conventional multichannel all-optical ring network which has an all-optical packet switching device 50. In both FIG. 1 and FIG. 2, the circles represent nodes, the thicker connecting lines are optical connectors, and the thinner connecting lines are electrical connectors. When a node 70 receives a wavelength division multiplexed optical signal from a previous node or a source node (N.sub.1. . . N.sub.n) through the optical cables 3, the received optical signal is demultiplexed by WDDM (Wavelength Division Demultiplexer) 10a. Then, one of the demultiplexed signals (about 10% of the received optical signal) is transmitted to an optical fiber coupler 4.
The optical fiber coupler 4 receives and splits the signal transmitted by WDDM so as to supply it to a laser diode optical amplifier 9. An optical address processor 7 consists of an optical fiber delay line matched filter (not shown) and a threshold detector (not shown). When the output signal of the amplifier 9 is supplied to the optical fiber delay line matched filter, the filter generates corresponding correlation pulses depending upon the incoming address signal and the node address information stored in the filter.
Since a peak value of the auto-correlation pulse is always higher than that of the cross-correlation pulse, the threshold detector easily determines whether the address of the incoming cell matches the node's own address. If an address match is found, the optical fiber delay line matched filter transmits auto-correlation pulses to the threshold detector. If, however, the address match is not found, the filter transmits cross-correlation pulses to the threshold detector. Thus, according to whether the maximum value of the correlation pulses is larger than the threshold value or not, the threshold detector generates a digital control signal of the first level (e.g. a logic "1" state) or of the second level (e.g. a logic "0" state) to drive the gate pulse generator 1 which controls an optical switch 6.
In other words, when the digital control signal of the logic state "1" is generated by the detector (i.e. when the destination address of the incoming cell matches the present node's own address), the optical switch 6 transmits the incoming optical signal to the receiver of the present node. However, when the digital control signal of the logic state "0" is generated, the optical signal is bypassed to the laser diode optical amplifier 9 where the incoming optical signal is amplified. Then, the output signal of the amplifier 9 is transmitted to the WDM (Wavelength Division Multiplexer 10 through the optical fiber coupler 4 in order to be multiplexed with optical signals output by other channels. Thereafter, the resultant signal is transferred to the next node.
As described above, since the conventional optical ring network uses a unidirectional ring structure, even when a node transmits a signal to a neighboring node located on the opposite side of the transmission travelling from the first node, the signal must continue to travel in the same direction around the ring to reach the neighboring node on the other side. As a result, the signal propagation time between the source node and destination node is nearly equal to the time required for one complete circuit of the ring cable.